Introduction
Argentina is the second largest country in South America, with a total area of approximately 2.7 million km². Due to its vast area and geographical location in the southernmost portion of South America, it presents a large diversity of habitats distributed in 16 biogeographical provinces (Arana et al., Reference Arana, Natale, Ferretti, Romano, Oggero, Martínez, Posadas and Morrone2021). Said diversity ranges from high mountainous environments to lowlands that include very varied continental ecosystems, from warm rainforests in the north to southern subantarctic forests in the south, and from dry deciduous forests to vast floodplains and deserts (Dos Santos et al., Reference Dos Santos, Domínguez, Miranda, Gutiérrez Gregoric and Cuezzo2020).
Environmental heterogeneity favours the development of a diverse fauna, as well as a great biodiversity of parasites with different life cycles, especially digeneans. Digeneans typically have complex life cycles that involve three hosts – ▫a definitive host (generally a vertebrate), a first intermediate host (mainly a mollusc), and a second intermediate host that can be an invertebrate (e.g., mollusc, annelid and crustacean) or a vertebrate (e.g., fish and amphibian); and several developmental stages (eggs, miracidia, sporocysts, rediae, cercariae, metacercariae and adults). In general, most of the life cycle takes place in aquatic environments. The first intermediate host (freshwater or marine snail) is specific, and provides the resources for development and asexual reproduction, and the means by which trematodes can reach their next host (Lockyer et al., Reference Lockyer, Jones, Noble and Rollinson2004). The larval stages in the first intermediate host can be used as positive indicators of the host community. They are functioning trophic links that are essential in determining life cycles. The study of larval digeneans is relevant both for their biodiversity and for their medical and veterinary importance. These larval stages can cause several diseases in humans and domestic animals such as clonorchiasis, paragonimiasis, schistosomiasis, fascioliasis and cercarial dermatitis (Loker & Hofkin, Reference Loker and Hofkin2015). In Argentina, fascioliasis and cercarial dermatitis are common parasitic zoonoses (Veleizán et al., Reference Veleizán, Flores and Viozzi2016; Carnevale et al., Reference Carnevale, Malandrini, Pantano, Sawicki, Kamenetzky, Soria and Velásquez2021).
The first studies in Argentina were carried out by Wernicke in the 1880s (Reference Wernicke1888, Reference Wernicke1892), and cited by Ostrowski de Núñez (Reference Ostrowski de Núñez1977a); unfortunately, these works were not available. The following record of cercariae was by Bacigalupo (Reference Bacigalupo1930, Reference Bacigalupo1932), who described the emergence of the cercariae of Fasciola hepatica from the snail Galba viatrix in the Buenos Aires province. In the 1950s, Lothar Szidat described the first cercariae from Argentina and pioneered the study of helminths in the country. His contribution to the knowledge about larval digeneans was very important, and included taxonomic descriptions, identification of parasite life histories, and definition of human and animal diseases, among others (Ostrowski de Núñez, Reference Ostrowski de Núñez1994). Subsequently, his successor, the Argentinian researcher Margarita Ostrowski de Núñez, described more than 100 larval stages of digeneans and elucidated the life cycles of about 30 species of trematodes in fish, reptiles, birds and mammals from freshwater environments. This was a significant contribution to the knowledge about the biology and taxonomy of this group of parasites. Since then, various authors have reported numerous larval stages and life cycles of digeneans. Based on the literature, the present checklist catalogues the diversity of larval digeneans in molluscs from inland waters of Argentina. It includes information on all digeneans, hosts, localities, habitats, prevalence values when available, life cycle if known, and references, among other data.
Materials and methods
An exhaustive search was performed on most of the published records on digeneans in molluscan hosts from freshwater and estuarine habitats in Argentina. The search was carried out in databases and electronic resources such as Google Scholar, Scopus, Research Gate, personal databases, through communication with different authors, and by acquiring earlier paper articles in scientific libraries. The search terms included in English, and in Spanish and German (within parentheses) were larval trematodes (trematodes larvales/Trematoden Larven), larval digeneans (digeneos larvales/Digene Larven), cercaria (cercaria/Cercarien), metacercaria (metacercaria/Metacercarie), molluscs (moluscos/Mollusken), freshwater (agua dulce/Süßwasser), and estuary (estuario/Ästuar) along with the term Argentina. All sources of metadata were consulted up to June 2021. The compiled data were used to create a spreadsheet. The superfamily and family of the digeneans found were determined following Gibson et al. (Reference Gibson, Jones and Bray2002), Jones et al. (Reference Jones, Bray and Gibson2005), Bray et al. (Reference Bray, Gibson and Jones2008) and the World Register of Marine Species (2021). The species of molluscan hosts were updated according to Rumi et al. (Reference Rumi, Gutiérrez Gregoric, Núñez, César, Roche, Tassara, Martin and Armengol2006, Reference Rumi, Gutiérrez Gregoric, Núñez and Darrigran2008) and Mollusca Base eds. (2021). The names of the superfamily, family, and species of digeneans and the names of molluscan host species were taken from the published articles or theses. In cases where the name is no longer accepted, it was modified according to the bibliography and electronic databases mentioned above. The ecoregions of Argentina were referred to as in Arana et al. (Reference Arana, Natale, Ferretti, Romano, Oggero, Martínez, Posadas and Morrone2021).
The checklist of digeneans found in molluscs from Argentina is presented in alphabetical order for each taxonomic category. It also includes: first intermediate molluscan host species; location of mollusc collection in Argentina, along with geographical references; prevalence values (when available); habitat type of the respective larval digenean (lake, shallow lake, pond, temporary pond, lagoon, estuary, reservoir, creek, stream, river, flooded areas of the river and rice field); second intermediate and definitive hosts (when available); and references in chronological order. Most records were based on naturally infected molluscs. Cases that proved to be susceptible to experimental infection but were not found to be naturally infected were denoted with a ‘^’ symbol. Cases that proved to be susceptible to experimental infection and found to be naturally infected were denoted with a ‘<’ symbol.
Results
A total of 113 scientific articles and nine theses were analysed. The studies, published between 1930 and 2021, reported the interaction between digeneans and molluscs from freshwater and estuarine habitats in Argentina. The 34 mollusc species reported belonged to 17 genera (table 1) and were parasitized by 213 larval digeneans from 13 superfamilies and 35 families (table 2). All these digeneans were identified in 102 locations corresponding to eight of the 16 biogeographical provinces in Argentina (fig. 1 and supplementary table S1). Only 45 (21.1%) digeneans were identified to species level and 18 (8.5%) to generic level. In addition, only 10 species (4.7%) have genetic sequences deposited in the GenBank. The most represented family was Echinostomatidae, with 36 records. It is followed by the families Schistosomatidae (18 taxa), Heterophyidae (17 taxa), Strigeidae (16 taxa), and Diplostomidae (14 taxa). A total of 40 out of 213 records have known the life cycle (13 natural cycles and 27 experimental cycles).
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20220506134408170-0316:S0022149X2100081X:S0022149X2100081X_fig1.png?pub-status=live)
Fig. 1. Map showing locations with the larval digeneans registered in the bibliographical survey in Argentina.
Table 1. List of molluscan hosts.
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20220506134408170-0316:S0022149X2100081X:S0022149X2100081X_tab1.png?pub-status=live)
Table 2. Digenean list found in molluscan hosts from Argentina.
![](https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20220506134408170-0316:S0022149X2100081X:S0022149X2100081X_tab2.png?pub-status=live)
a material deposited.
b genetic sequence deposited.
c experimental host.
d natural and experimental host.
Of the total molluscan hosts, 31 were gastropods and three bivalves. Among the gastropods, 18 species were first intermediate hosts only, 12 were first and second intermediate hosts, five were second intermediate hosts only, and one species was first and definitive host. With regard to the bivalves, two acted as first intermediate hosts, and one as second intermediate host. The molluscs of the genera Heleobia and Biomphalaria showed the highest levels of infection and the highest number of digeneans (table 1). The other mollusc genera acted as hosts of less than 10 digenean species.
Discussion
In Argentina, the first studies on larval stages of digeneans using freshwater snails were carried out almost 100 years ago. The first record included in this checklist was made by Bacigalupo (Reference Bacigalupo1930) on F. hepatica. Later, Szidat (Reference Szidat1951) described a new cercaria species. Both studies involved digenean species of zoonotic importance in the region. As shown in this checklist, subsequent studies have described 213 larval digeneans belonging to 35 families. Historically, taxonomic determination has been made based on morphology and it has been assumed that digeneans were specific to first intermediate hosts. However, we consider that the parasites described and reported here probably correspond to more than 213 species of digeneans. There is possible plasticity in morphological characters across hosts in different environments, as well as the presence of cryptic species in a snail host, and the possibility of multiple mollusc species as first intermediate hosts.
This study is the second checklist of larval digeneans in molluscan hosts in Argentina. The first one focused only on marine digeneans and reported 31 species of parasites in 20 species of molluscs (Bagnato et al., Reference Bagnato, Gilardoni, Di Giorgio and Cremonte2015). In addition, two previous checklists recorded larval digeneans in both marine and freshwater molluscan hosts in South America. In Chile, four digenean species have been identified in the freshwater snail Chilina dombeyana, and five species in marine gastropods and bivalves (Muñoz & Olmos, Reference Muñoz and Olmos2008). In Brazil, Pinto & De Melo (Reference Pinto and De Melo2013) recorded 102 cercaria species in 60 molluscan species.
Based on this research, three aspects should be improved in the study of larval digeneans in freshwater environments, namely distribution, use of taxonomic tools and host taxonomy. Firstly, the distribution of records is uneven; records were available for only eight of the 16 biogeographical provinces proposed by Arana et al. (Reference Arana, Natale, Ferretti, Romano, Oggero, Martínez, Posadas and Morrone2021). This could be explained by several factors, namely the extent of the Argentine territory, the scarce funding allocated to biodiversity, and the cost of travel. Thus, data are mainly recorded close to the centres where researchers carry out their activities. In addition, some water bodies are located in private fields or protected areas that are difficult to access. The second aspect relates to the use of current molecular tools to determine the gene sequences of cercariae. As mentioned above, only 21.1% of the cercariae found have been classified to species level and only 4.7% have been sequenced. Therefore, it is necessary to increase the use of molecular tools to identify cercariae to species level, determine the existence of cryptic species and better understand the biology of this group. Finally, the third aspect to improve is the diversity of hosts examined. According to Rumi et al. (Reference Rumi, Gutiérrez Gregoric, Núñez and Darrigran2008), 166 species of freshwater molluscs (101 gastropod and 65 bivalve species) have been reported in Argentina, but only 34 gastropod and three bivalve species have been examined for parasites over almost 100 years. There should be more collaboration with malacologists to improve the identification of both the larval digeneans and their molluscan hosts.
It is noteworthy that all of the authors included in the checklist who carried out studies in freshwater and marine environments in South America (Muñoz & Olmos, Reference Muñoz and Olmos2008; Pinto & De Melo, Reference Pinto and De Melo2013; Bagnato et al., Reference Bagnato, Gilardoni, Di Giorgio and Cremonte2015) have mentioned these three aspects. In addition, the procedure of collecting, describing and classifying cercariae is time-consuming and requires both training and adequate equipment. Therefore, in order to deepen the knowledge about the diversity and distribution of digeneans, researchers should work together, unifying efforts and minimising costs.
Considering the limited knowledge about life cycles of digeneans, the information presented here may have wider implications, mainly for neighbouring countries to Argentina such as Chile, Uruguay, Paraguay, Bolivia and Brazil. Most of the molluscan hosts recorded are also present in these countries, and some of them are of health and zoonotic interest. For instance, the genus Biomphalaria is found in Brazil (Ciccheto et al., Reference Ciccheto, Stabile, Fermino, Fabrin, de Oliveira, Takemoto and da Graça2021; Mesquita et al., Reference Mesquita, dos Santos Neves, Scholte, dos Santos Carvalho, Fonseca and Caldeira2021; among others) and F. hepatica is present in Bolivia and Brazil (Mas-Coma et al., Reference Mas-Coma, Buchon, Funatsu, Angle, Artigas, Valero and Bargues Castelló2020; Schwantes et al., Reference Schwantes, Quevedo, D’Ávila, Molento and Graichen2020; Silva et al., Reference Silva, Freitas, Dutra and Molento2020; among others). Moreover, some of the birds and mammals that act as definitive hosts, such as Otaria flavescens in Chile and Brazil (Pereira et al., Reference Pereira, Müller, Secchi, Pereira and Valente2013; Hermosilla et al., Reference Hermosilla, Hirzmann, Silva, Scheufen, Prenger-Berninghoff, Ewers and Taubert2018), and Laurus dominicanus in Chile (Gonzalez-Acuna et al., Reference Gonzalez-Acuna, Cerda, Lopez, Ortega, Mathieu and Kinsella2009), are known to migrate in the southern part of the American continent. Cosmopolitan host species such as Physella acuta (Lawton et al., Reference Lawton, Allan, Hayes and Smit2018; Pantoja et al., Reference Pantoja, Faltýnková, O'Dwyer, Jouet, Skírnisson and Kudlai2021) and migratory bird species from other continents are also in the checklist, making the present work of potential global interest. For instance, many migratory shorebirds of the families Charadriidae and Scolopacidae breed in the tundra of the Northern Hemisphere, and then migrate southwards to spend the non-breeding period in coastal and inland wetlands of South and Central America. These bird families have been reported in inland wetlands and temporary artificial wetlands such as flooded rice fields in northeastern Argentina (Blanco et al., Reference Blanco, López-Lanús, Dias, Azpiroz and Rilla2006; Blanco & De la Balze, Reference Blanco and De la Balze2011).
In view of the presence of common intermediate and definitive hosts and the biogeographical characteristics shared by South American countries, it is necessary to carry out studies at the regional level to understand the factors that determine the spatial and temporal dynamics of digeneans, as well as to elucidate their life cycles. Furthermore, larval digenean assemblages in freshwater molluscs should be studied in greater depth, as our results reveal a fertile field for future studies on taxonomy, diversity conservation, ecology, health and zoonoses. In addition, molecular tools should be used to complement morphological descriptions for better taxonomic identification.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/S0022149X2100081X
Acknowledgements
The authors thank Matías Paredes Davies for his collaboration in the preparation of the distribution map, Florencia Liquín for her help in the preparation of the first table (data from Salta), and to the Asociación Parasitológica Argentina for the space provided for the first freshwater digeneans conference. A special thanks to Margarita Ostrowski de Núñez for her comments and helpfulness to send the bibliography, and especially for all the work done in the determination of larval digeneans in Argentina.
Financial support
Financial support was received from Universidad Nacional de Salta (Grant number 2481/0 CIUNSa to D.D.), from ANPCyT (PICT 2017- 1819 to J.A.E.), from Universidad Nacional de Mar del Plata (Grant number 15/E935 EXA997/20 to J.A.E) and from Universidad Nacional del Comahue (Grant number B225 to V.F.)
Conflicts of interest
None.
Ethical standards
Not applicable.
Authors’ contribution
All authors contributed to the work conceiving, conducting data gathering, performing statistical analyses and writing the article.